The invention relates to flat panel displays and in particular to thin film devices for a flat panel displays, having different electrical characteristics on the driving circuit region and the pixel region.
The demand for active-matrix flat panel displays (FPDs) has increased rapidly in recent years. Currently techniques for forming thin film transistors (TFTs) are needed. In conventional TFT fabrication, an amorphous silicon film formed on a glass substrate is typically used as a channel or active layer of the transistor.
Compared with polysilicon film, amorphous silicon film has a smaller grain size and irregular grain arrangement, reducing the electron mobility and thus lowering the performance of the TFT. Generally, the electron mobility of polysilicon film is 100 times higher than that of amorphous silicon film. In semiconductor fabrication, polysilicon film is typically prepared by low pressure chemical vapor deposition (LPCVD) and then annealed at a temperature above 900° C. The same method cannot be employed in the FPD industry as the strain temperature of glass is about 650° C. Thus, low temperature polysilicon (LTPS) is a novel application for FPD technology. Moreover, LTPS integrates driving circuits on a glass substrate having pixels thereon, reducing the manufacturing cost.
In an active-matrix organic light emitting display (AMOLED), however, the electrical characteristics of the TFTs on the driving circuit region are different from that on the pixel region. For example, in the driving circuit region, the p-type thin film transistor (PTFT) has an absolute threshold voltage value substantially equal to that of an n-type thin film transistor (NTFT). In the pixel region, the threshold voltage of the PTFT serving as an OLED switching element must be lower to reduce power consumption and extend the lifetime thereof. Accordingly, in LTPS technology, it is difficult to form transistors with the desired electrical characteristics on the driving circuit region and the pixel region at the same time.